Ask a pro: Vapour barrier

Expanded polystyrene (the white stuff) is typically not vapour-impermeable. The polyethylene-film vapour barrier that covers the batt insulation above should continue down over the foam. Technically, the wood strapping that contacts the masonry should also be separated from it by a damp-proofing barrier of some kind. When you get your permit, check with the building department to determine the acceptable damp-proofing in your area. Tar-saturated building paper may be acceptable, and there are a host of liquid roll-on or paint-on membranes that may help your basement wall system to perform optimally.